text: A model put forward by Lee Kump, Alexander Pavlov and Michael Arthur in 2005 suggests that oceanic anoxic events may have been characterized by upwelling of water rich in highly toxic hydrogen sulfide gas, which was then released into the atmosphere. This phenomenon would probably have poisoned plants and animals and caused mass extinctions. Furthermore, it has been proposed that the hydrogen sulfide rose to the upper atmosphere and attacked the ozone layer, which normally blocks the deadly ultraviolet radiation of the Sun. The increased UV radiation caused by this ozone depletion would have amplified the destruction of plant and animal life. Fossil spores from strata recording the Permian–Triassic extinction event show deformities consistent with UV radiation. This evidence, combined with fossil biomarkers of green sulfur bacteria, indicates that this process could have played a role in that mass extinction event, and possibly other extinction events. The trigger for these mass extinctions appears to be a warming of the ocean caused by a rise of carbon dioxide levels to about 1000 parts per million.